502 



BOTANY 



have the green tissue restricted to the stems, which are, moreover, 

 provided with an impervious, thick epidermis. 



Another type of xerophyte is seen in such forms as the Agaves 

 (PI. IX) and Aloes, where the stem is very short and the leaves 

 enormously enlarged, and their inner cells gorged with water, so that 

 they serve as reservoirs of moisture protected by the very heavy 

 epidermis. An admirable example of this type is seen in the Century- 

 plant, which inhabits the deserts of Arizona and Mexico. 



Cacti. Perhaps the most perfect type of xerophytes are the Cacti, 

 especially characteristic of the arid regions of the southwestern 

 United States and Mexico, and the very similar Euphorbias of Africa. 

 In these plants the leaves are usually entirely suppressed, and the 

 greatly thickened stems expose a relatively small surface to the air. 

 The stems may branch freely, but sometimes the stem forms a glob- 

 ular or oblong solid body, which exposes a minimum surface to the 



atmosphere, and its surface is 

 so thoroughly protected, and 

 the supply of water in the mass 

 of soft tissue within the stem 

 is so great, that the plant may 

 be exposed for weeks to the 

 fierce rays of the sun without 

 appreciable loss of water. 



The surface of the leaves 

 and stems in these xerophytic 

 plants is very generally covered 

 with a coating composed either 

 of a waxy secretion forming 

 a grayish coating or " bloom " 

 upon the surface, or it is pro- 

 tected by a mass of hairs, which 

 serve as a screen against the 

 too active light rays. There 

 is often, also, a layer of color- 

 less hypodermal cells between 

 the green mesophyll and the 

 epidermis, which hold water 

 and screen the cells beneath. 

 A similar protection against the force of the sun's rays is offered 

 by the leaves of certain xerophytes, which are placed vertically, in- 

 stead of in the normal horizontal position. This is seen in some 

 species of Arctostaphylos, the "Manzanita" of the California moun- 

 tains (Fig. 470), and still more markedly in most species of Eucalyp- 

 tus. In the Blue-gum, E. globulus, the first leaves are horizontal, 

 but are gradually replaced by the sickle-shaped mature leaves, which 



FIG. 470. Arctostaphylos Manzanita. With 

 leaves placed vertically as a protection 

 against too intense sunshine. 



